Method For Trapping CO2 By Solid Cryocondensation In A Turbine

ABSTRACT

The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.

The present invention relates to a method of capturing carbon dioxide ina fluid comprising at least one compound more volatile than carbondioxide CO2, for example methane CH4, oxygen O2, argon Ar, nitrogen N2,carbon monoxide CO, helium He and/or hydrogen H2.

The invention can be notably applied to units producing electricityand/or steam from carbon fuels such as coal, hydrocarbons (natural gas,fuel oil, petrochemical residue, etc.), household waste, biomass but canalso be applied to gases from refineries, chemical plants, steel-makingplants or cement works, to the treatment of natural gas as it leavesproduction wells. It could also be applied to the flue gases fromboilers used to heat buildings or even to the exhaust gases fromtransport vehicles, and more generally to any industrial process thatgenerates CO2-containing flue gases.

Carbon dioxide is a greenhouse gas. For environmental and/or economicreasons, it is becoming increasingly desirable to reduce or eveneliminate discharges of CO2 into the atmosphere by capturing it andthen, for example, storing it in appropriate geological layers or byrealizing it as an asset in its own right.

A certain number of techniques for capturing carbon dioxide, for examplemethods based on scrubbing the fluids with solutions of compounds thatseparate the CO2 by chemical reaction, for example scrubbing using MEA,are known. These methods typically have the following disadvantages:

-   -   high energy consumption (associated with the regeneration of the        compound used to react with the CO2),    -   degradation of the compound that reacts with the carbon dioxide,    -   corrosion due to the compound reacting with the carbon dioxide.

In the field of cryo-condensation, that is to say of cooling until solidCO2 appears, mention may be made of document FR-A-2820052 whichdiscloses a method allowing CO2 to be extracted by anti-sublimation,that is to say by solidification from a gas without passing via theliquid state. The cold required is provided by means of fractionateddistillation of refrigerating fluids. This method consumes a great dealof energy.

Document FR-A-2894838 discloses the same type of method, with some ofthe liquid CO2 produced recirculated. The cold may be supplied byvaporizing LNG (liquefied natural gas). This synergy reduces thespecific energy consumption of the method, although this remains highdespite this, and requires an LNG terminal.

Document U.S. Pat. No. 3,614,872 describes a separation method in whichthe adiabatic and isentropic expansion of the carbon dioxide yields arefrigerating fluid.

It is one object of the present invention to provide an improved methodof capturing carbon dioxide from a fluid containing CO2 and at least onecompound more volatile than the latter.

The invention relates first of all to a method for producing at leastone CO2-lean gas and one or more CO2-rich primary fluids from a processfluid containing CO2 and at least one compound more volatile than CO2and implementing:

-   a) a first cooling of said process fluid by exchange of heat with no    change in state;-   b) a second cooling of at least part of said process fluid cooled in    step a) so as to obtain at least one solid containing predominantly    CO2 and at least said CO2-lean gas; and-   c) a step comprising liquefaction and/or sublimation of at least    part of said solid and making it possible to obtain said one or more    CO2-rich primary fluids;    said method being characterized in that step b) is performed in at    least one expansion turbine, said solid forming inside said turbine.

The process fluid generally comes from a boiler or any plant thatproduces flue gases. These flue gases may have undergone variouspre-treatments, notably with a view to removing NOx (oxides ofnitrogen), dust, SOx (oxides of sulfur) and/or water.

Prior to separation, the process fluid is either monophasic, in gaseousor liquid form, or polyphasic. What is meant by “gaseous” form is“essentially gaseous” form, that is to say that it may notably containdust, solid particles such as soot and/or droplets of liquid.

The process fluid contains CO2 that is to be separated from the otherconstituents of said fluid by cryo-condensation. These otherconstituents comprise one or more compounds more volatile than carbondioxide in terms of condensation, for example methane CH4, oxygen O2,argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.The process fluids generally comprise predominantly nitrogen orpredominantly CO or predominantly hydrogen.

The process fluid generally comes from a boiler or any plant thatproduces flue gases. These flue gases may have undergone variouspre-treatments, notably with a view to removing NOx (oxides ofnitrogen), dust, SOx (oxides of sulfur) and/or water.

Prior to separation, the process fluid is either monophasic, in gaseousor liquid form, or polyphasic. What is meant by “gaseous” form is“essentially gaseous” form, that is to say that it may notably containdust, solid particles such as soot and/or droplets of liquid.

The process fluid contains CO2 that is to be separated from the otherconstituents of said fluid by cryo-condensation. These otherconstituents comprise one or more compounds more volatile than carbondioxide in terms of condensation, for example methane CH4, oxygen O2,argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.The process fluids generally comprise predominantly nitrogen orpredominantly CO or predominantly hydrogen.

In step a) the process fluid is first of all cooled without a change instate. This cooling may advantageously take place at least in part byexchange of heat with CO2-rich fluids from the separation process. Inaddition or as an alternative, it may advantageously take place at leastin part by exchange of heat with the CO2-lean gas from the separationprocess. These cold fluids from the separation process are heated up,while the process fluid is cooled down. This makes it possible to reducethe amount of energy required for the cooling operation.

Step b) consists in solidifying the initially gaseous CO2 by raising theprocess fluid to a temperature below the triple point for CO2 while thepartial pressure of the CO2 in the process fluid is below that of thetriple point for CO2. For example, the total pressure of the processfluid is close to atmospheric pressure. This solidification operation issometimes known as “cryo-condensation” or “anti-sublimation” of the CO2and, by extension, of the process fluid.

According to one particular embodiment, all the components of theprocess fluid which do not solidify in step a) or which are not lumpedtogether with the solid CO2, remain in the gaseous state. Theseconstitute the CO2-lean gas.

Certain compounds more volatile than CO2 do not solidify and remain inthe gaseous state. Together with the non-solidified CO2 these willconstitute said CO2-lean gas, that is to say will constitute said gasthat comprises less than 50% CO2 by volume and preferably less than 10%CO2 by volume. According to one particular embodiment, said CO2-lean gascontains less than 1% CO2 by volume. According to another particularembodiment, it contains more than 2% thereof. According to anotherparticular embodiment, it contains more than 5% thereof. A solidcomprising predominantly CO2, that is to say containing at least 90% byvolume if considered in the gaseous state, preferably containing atleast 95% by volume, and more preferably still containing at least 99%CO2 by volume, is formed.

This solid may comprise other compounds than CO2. Mention may, forexample, be made of other compounds which might also have solidified, oralternatively of bubbles and/or drops of fluid contained within saidsolid lump. This explains how the solid could potentially consist of notonly solid CO2. This “solid” may contain non-solid parts such as fluidinclusions (drops, bubbles, etc.).

This solid is then isolated from the compounds that have not solidifiedafter cryo-condensation and recovered. Next, in step c), it is returnedto temperature and pressure conditions such that it changes into afluid, liquid and/or gaseous, state. At least part of said solid maythen liquefy. This then gives rise to one or more CO2-rich primaryfluids. These fluids are said to be “primary” to distinguish them fromtreatment fluids which are said to be “secondary”. What is meant by“CO2-rich” is something “comprising predominantly CO2” within themeaning defined hereinabove.

The inventors have demonstrated that it is particularly advantageous tocarry out the first and/or the second cooling of the process fluid usingone or more refrigerating cycles each comprising at least onenear-isentropic expansion of a gas. These refrigerating cycles consistof several steps which cause a so-called “working” fluid to pass viaseveral physical states characterized by given composition, temperature,pressure, etc. conditions. The presence, among the steps of the cycle,of at least one near-isentropic expansion, that is to say of anexpansion that causes the entropy of the expanded fluid to increase byless than 25%, preferably less than 15% and more preferably still, lessthan 10% makes it possible to improve the energy consumption of theseparation process. By convention, entropy is considered to be zero at atemperature of zero K (kelvin).

Depending on circumstances, the method according to the invention maycomprise one or more of the following features:

-   -   said solid is in the form of carbon dioxide snow.    -   said expansion turbine comprises at least one rotor part and at        least one stator part situated upstream of said rotor part, and        in that step b) is performed in said rotor part. Said rotor part        is the region situated between the leading edge and the trailing        edge of the rotor vanes; it comprises the volume displaced by        the rotor as it rotates.    -   said process fluid comprising CO2 and at least one compound more        volatile than CO2 remains in the vapor state in said stator        part.    -   said process fluid is in the supersaturated vapor state in said        stator part situated upstream of said rotor part.    -   said expansion turbine is a centripetal radial turbine or a        centrifugal radial turbine or an axial turbine or a shock wave        supersonic turbine.    -   the method further comprises after step b), a step b1) in which        said solid comprising predominantly CO2 and said CO2-lean gas        that were obtained in step b) are separated into at least one        CO2-rich fraction and a CO2-lean fraction, said step b1) being        performed in a region situated downstream of said rotor part.        “Downstream of said rotor part” means downstream of the trailing        edge of the rotor vanes.    -   in step b), a rotational movement is imparted to said solid        comprising predominantly CO2 and to said CO2-lean gas and, in        step b1), the separation of said solid containing predominantly        CO2 from said CO2-lean gas, both obtained in step b), comprises        a separation through a centrifugal effect induced by said        rotational movement imparted, in step b), to said solid        comprising predominantly CO2.    -   certain parts of said turbine are heated. These parts are, for        example, those in contact with the fluid of the stator part        upstream of the rotor, of the rotor part and/or of the stator        part downstream of the rotor.    -   said turbine comprises surfaces that are polished and/or that        are coated with a given material aimed at limiting heterogeneous        nucleation of said solid comprising predominantly CO2 on said        surfaces and/or on said coating material.    -   said process fluid comprises compounds less volatile than CO2        and in that said method comprises, prior to step b), one or more        steps of purifying said process fluid (40) to remove said less        volatile compounds. If these compounds were not eliminated, they        would carry the risk of forming solid compounds at temperatures        higher than the solid cryo-condensation temperature for CO2 and        therefore of potentially initiating earlier nucleation of the        CO2.    -   said compounds less volatile than CO2 include H20, SO2, NxOy and        the solid compounds present in the process gas at ambient        temperature.    -   the solid compounds at levels of below 1 mg/m3, preferably below        100 μg/m3, are eliminated from the process fluid (in the unit        103).    -   said turbine comprises titanium.    -   the materials used in the mass of the turbine or as coatings are        resistant to erosion by solid particles comprising predominantly        CO2. Use may, for example, be made of titanium.    -   a sweeping gas is injected into said turbine, said sweeping gas        coming into contact with said process fluid. This sweeping gas        may, for example, be air or nitrogen and prevents solid CO2 from        being present across the back of the rotor part; if solid were        to accumulate in this region it could erode the rotor part.    -   said turbine has a degree of reaction in excess of 50%, that is        to say more than 50% of the drop in enthalpy of the fluid (the        difference in enthalpy between the inlet and outlet of the        turbine) occurs in the rotor part.

Moreover, step b1) may use two concentric cones intended to recover saidCO2-rich fraction and said CO2-lean fraction. Alternatively, in stepb1), said solid comprising predominantly CO2 and said CO2-lean gas bothobtained in step b) can also be decelerated and then separated into saidCO2-rich fraction and said CO2-lean fraction under the effect ofgravity.

To provide another part of the cold required to carry out the firstand/or second coolings, recourse may be had to one or more cyclescomprising an expansion of a fluid that is not a near-isentropicexpansion, for example reverse-Rankine cycles. These cycles are said tobe reversed because they are used as refrigerating cycles. Theirbenefit, as a supplement to the refrigerating cycles employingnear-isentropic expansion, is that they do not require a large amount ofworking fluid. By contrast, they are less energy-efficient.

According to one embodiment, some of the near-isentropic expansions ofthe refrigerating cycle or cycles provide work.

The working fluids may be of varying kinds. According to variousembodiments, these fluids may comprise nitrogen and/or argon. They mayalso comprise all or part of the CO2-lean gas obtained or of the processfluid. These fluids may be mixed with other fluids or have undergoneintermediate steps of compression, expansion, etc.

When the working fluid of the refrigerating cycle comprises all or partof the process fluid, the near-isentropic expansion or expansions thatdo not provide external work may give rise to a cooling of the workingfluid such that solid CO2 appears. This may constitute all or part ofthe second cooling of the process fluid. According to one particularembodiment, these near-isentropic expansions are carried out through aVenturi (a throat with Venturi effect).

The abovementioned causing of the fluid to rotate can be obtained by anyconventional means, for example by suitably oriented vanes. The increasein speed is achieved through a Venturi effect. The temperature of theworking fluid drops. Solid particles of CO2 appear. The fluid has arotational movement about an axis substantially parallel to thedirection of the flow, like a corkscrew. This creates a centrifugaleffect allowing these solid particles to be recovered at the peripheryof the flow.

According to a preferred embodiment, any work that might be produced bythe near-isentropic expansion or expansions serves in part to compressthe fluids in other steps of the method.

The invention also relates to the method applied to industrial fluegases with a view to capturing CO2.

According to one particular embodiment, these flue gases come from aplant producing energy (steam, electricity) and may have undergonepretreatments.

Other specifics and advantages will become apparent from reading thefollowing description given with reference to the figures in which:

-   -   FIG. 1 schematically depicts a plant employing a method for        purifying CO2 according to the invention, with a refrigerating        cycle employing an auxiliary fluid as working fluid,    -   FIG. 2 schematically depicts part of a plant employing an        alternative form of this method, with a refrigerating cycle        using the CO2-lean gas by way of working fluid and comprising a        near-isentropic expansion with the production of work,    -   FIG. 3 schematically depicts part of a plant employing another        alternative form of this method, with a refrigerating cycle        using the CO2-lean gas as working fluid and comprising a        near-isentropic expansion with the production of work,    -   FIG. 4 schematically depicts part of a plant employing an        alternative form of the method with a refrigerating cycle using        the process fluid as working fluid and comprising a        near-isentropic expansion with the production of work, during        which there is no cryo-condensation of CO2,    -   FIG. 5 schematically depicts part of a plant employing an        alternative form of the method with a refrigerating cycle using        the process fluid as working fluid and comprising a        near-isentropic expansion with the production of work, during        which there is cryo-condensation of CO2,    -   FIG. 6 schematically depicts part of a plant employing an        alternative form of the method with a refrigerating cycle using        the process fluid as working fluid and comprising a        near-isentropic expansion without the production of work, during        which there is cryo-condensation of CO2,    -   FIG. 7 schematically depicts part of a plant employing an        alternative form of the method, in which the second cooling also        comprises liquefaction and further comprising a refrigerating        cycle using the process fluid as working fluid and comprising        near-isentropic expansions without the production of work during        which expansions there is cryo-condensation of CO2.    -   FIG. 8 schematically depicts the use of a method according to        the invention in a plant for producing electricity on the basis        of coal with combustion in air.    -   FIG. 9 schematically depicts the use of a method according to        the invention in a plant for producing electricity on the basis        of coal with hybrid combustion or combustion in oxygen.    -   FIG. 10 schematically depicts the use of a method according to        the invention in a steel-making plant.    -   FIG. 11 schematically depicts the use of a method according to        the invention in a plant for producing synthesis gas operating        on oxygen.    -   FIG. 12 schematically depicts the use of a method according to        the invention in a plant for producing carbon monoxide from a        synthesis gas that comes from a steam reforming of a synthesis        gas.    -   FIG. 13 schematically depicts the use of a method according to        the invention with, on the one hand, a cycle for producing        energy using the cold of fusion of solid CO2 and, on the other        hand, additional purifications by distillation of the compounds        less volatile than CO2, then the compounds more volatile than        CO2.    -   FIGS. 14 and 15 depict a turbine for carrying out a        near-isentropic expansion of the process fluid with the        production of external work in accordance with the invention.

The plant illustrated in FIG. 1 implements the steps described below.

The fluid 24 consisting of flue gases is compressed in a compressor 101,notably to compensate for the pressure losses in the various pieces ofequipment in the plant. Let us note that this compression may also becombined with the compression known as the draft compression of theboiler that produces the flue gases. It may also be carried out betweenother steps of the method, or downstream of the CO2 separation method;

The compressed fluid 30 is injected into a filter 103 to eliminateparticles down to a level of concentration of below 1 mg/m³, preferablyof below 100 μg/m³.

Next, the dust-free fluid 32 is cooled to a temperature close to 0° C.,generally of between 0° C. and 10° C., so as to condense the water vaporit contains. This cooling is carried out in a tower 105, with waterinjected at two levels, the cold water 36 and water 34 at a temperatureclose to ambient temperature. It is also possible to conceive ofindirect contact. The tower 105 may or may not have packings.

The fluid 38 is sent to a unit that eliminates residual water vapor 107,for example using one and/or another of the following methods:

-   -   Adsorption on fixed beds, fluidized beds and/or rotary dryer,        the adsorbent potentially being activated alumina, silica gel or        a molecular sieve (3A, 4A, 5A, 13X, . . . );    -   Condensation in a direct-contact or indirect-contact exchanger.

The dried fluid 40 is then introduced into the exchanger 109 where thefluid is cooled down to a temperature close to, but in all events higherthan, the temperature at which CO2 solidifies. This temperature can bedetermined by a person skilled in the art aware of the pressure andcomposition of the process fluid 40. This temperature is situated ataround about −100° C. if the CO2 content of the process fluid is of theorder of 15% by volume and for a pressure close to atmospheric pressure.

The fluid 42 which has undergone a first cooling 109 is then introducedinto a vessel 111 where it continues to be cooled down to thetemperature that provides the desired level of CO2 capture.Cryo-condensation of at least part of the CO2 contained in the fluid 42occurs producing, on the one hand, a CO2-lean gas 44 and, on the otherhand, a solid 62 comprising predominantly CO2. The gas 44 leaves thevessel 111 at a temperature of the order of −120° C. This temperature ischosen as a function of the target level of CO2 capture. At thistemperature, the CO2 content of the gas 44 is of the order of 1.5% byvolume, namely a capture level of 90% starting out from a process fluidcontaining 15% CO2. There are various technologies that can be used forthis vessel 111:

-   -   Continous solid cryo-condensation exchanger in which solid CO2        is produced in the form of carbon dioxide snow, is extracted,        for example, using a screw and pressurized to introduce it into        a bath of liquid CO2 121 in which a pressure higher than the        triple point pressure for CO2 obtains. This pressurization can        also be carried out batchwise in a system of silos. Continuous        solid cryo-condensation may itself be performed in various ways:    -   Scraped surface exchanger, the scrapers for example being in the        form of screws to encourage extraction of the solid;    -   Fluidized bed exchanger so as to carry the carbon dioxide snow        along and clean out the tubes using particles for example of a        density greater than that of the carbon dioxide snow;    -   Exchanger in which solid is extracted by vibration, ultrasound,        a pneumatic or thermal effect (intermittent heating so as to        cause the carbon dioxide snow to fall);    -   Accumulation on a smooth surface with periodic “natural” fall        into a tank;    -   Batchwise solid cryo-condensation: in this case, several        exchangers in parallel can be used alternately. They are then        isolated, pressurized to a pressure higher than the triple point        pressure for CO2, so as to liquefy the solid CO2 and possibly        partially vaporize it.

The fluid 46 is then heated up in the exchanger 109. As it leaves, thefluid 48 can also be used notably to regenerate the unit used foreliminating residual vapor 107 and/or for producing cold water 36 byevaporation in a direct-contact tower 115 into which a dry fluid 50 isintroduced which then becomes saturated with water, vaporizing some ofit. The cold water could then potentially undergo additional cooling ina refrigerator unit 119.

The solid 62 comprising predominantly CO2 is transferred to a bath 121of liquid CO2.

This bath 121 needs to be heated in order to remain liquid, tocompensate for the addition of cold from the solid 62 (latent heat offusion and sensible heat). This can be done in various ways:

-   -   by exchange of heat with a hotter fluid 72. The cold energy from        the fluid 74 can be used elsewhere in the method,    -   by direct exchange, for example by tapping a fluid 80 from the        bath 121, heating it in the exchanger 109, and reinjecting it        back into the bath 121.

Liquid 64 comprising predominantly CO2 is tapped from the bath 121. Thisliquid is split into three streams. In the example, the first isobtained by an expansion 65 to 5.5 bar absolute producing a diphasic,gas-liquid, fluid 66. The second, 68, is obtained by compression 67, forexample to 10 bar. The third, 70, is compressed for example to 55 bar.The 5.5 bar level provides cold at a temperature close to the triplepoint temperature for CO2. The 10 bar level allows the transfer of thelatent heat of vaporization of the fluid 68 at around −40° C. Finally,at 55 bar, the fluid 70 does not vaporize during the exchange 109. Thereis efficient use to be made of the cold energy contained in the fluid 64during the exchange 109 while at the same time limiting the amount ofenergy required to produce a purified and compressed stream 5 of CO2.

Part of the cold required for the first cooling 109 and for the secondcooling 111 is provided by a refrigerating cycle 200 employing a workingfluid 51 which is argon. It comprises, in succession: a compression 129,possibly two compressions 56 and 57, a cooling by indirect exchange 109,a near-isentropic expansion 131 which gives rise to cooling, aheating-up in the vessel 111, and a heating-up 109. During the cooling109, part of the working fluid is tapped off then undergoesnear-isentropic expansion 130, followed by indirect exchange 109 andfinally compression 128 before reaching the compression stage 129. Thenear-isentropic expansions 130 and 131 supply work part of which can beused for the compressions 56 and 57.

This cycle 200 produces cold at between about −100 and −120° C. for thecryo-condensation 111 and between about 5° C. and −100° C. in order tooffset the deficit of cold during the exchange 109.

Another part of the cold needed for the first cooling 109 is provided byan additional refrigerating cycle 181, 183, for example of the reverseRankine type.

Another part of the cold needed for the second cooling 111 is providedby an additional refrigerating cycle 191, 193, for example of thereverse Rankine type.

Following the indirect exchange 109, the CO2-rich primary fluids 66, 68,70 are compressed in stages 141, 142, 143. For example, the first stagescompress gaseous streams. If need be, the compressed CO2 75 is cooled byan indirect-contact exchanger to convert it to liquid form. It is thenmixed with the stream 73. This liquid mixture is pumped to the transportpressure (fluid 5). As the transport pressure is generallysupercritical, the supercritical fluids will, by extension, beconsidered to be liquid at a temperature below that of the criticalpoint for CO2.

FIGS. 2 to 7, which depict examples according to particular embodimentsof the invention, do not depict the steps which apply to the processfluid 40 prior to its first cooling 109, nor do they depict thecompression of the CO2-rich primary fluids after the exchange of heat109. They depict only changes by comparison with FIG. 1 relatingessentially to the refrigerating cycles that provide the cold for theexchanges 109 and 111.

FIG. 2 illustrates an alternative form of the near-isentropic expansionwith production of work, in which the working fluid is the CO2-lean gas44. The cryo-condensation method is the same as in FIG. 1. Only thechanges are detailed below.

The CO2-lean gas 44 is compressed, for example by a multi-stagecompressor 315. On leaving, the fluid 303 is cooled if necessary to theinlet temperature for the exchanger 109 by the exchanger 316. This maybe a direct-contact or an indirect-contact exchanger.

The compressed CO2-lean gas 304 is cooled in the exchanger 109 so thatit can be expanded in the turbine 312 (near-isentropic expansion) so asto provide some of the cold needed for the exchange 111. The fluid 307leaving the exchanger 111 is once again expanded (near-isentropicexpansion) to provide work and cold for the exchanger 111 via the fluid308. This loop in which the CO2-lean gas is expanded can be repeated asmany times as necessary.

After the exchanger 111, the CO2-lean gas 46 is heated up in theexchanger 109. The outgoing fluid 48 is processed like the fluid 48 inFIG. 1.

Some of the cold needed for the exchanger 111 may be supplied by arefrigerating cycle 191, 193 of the Rankine type.

FIG. 3 illustrates another alternative form of the near-isentropicexpansion with the production of work.

The CO2-lean gas 44 gives up cold energy in the exchangers 111 and 109.It is then compressed by the multi-stage compressor 415. Next, it iscooled if necessary to the inlet temperature of the exchanger 109 in theexchanger 416. This may be a direct-contact or an indirect-contactexchanger.

The CO2-lean gas 404 is once again cooled in the exchanger 109 before itis being expanded by the turbine 412. This near-isentropic turbineproduces the cold required to compensate for part of the deficit of coldenergy in the exchanger 111.

Next, the fluid 407 is expanded again by the near-isentropic turbine414. The fluid 408 gives up its cold energy to compensate for part ofthe deficit of cold energy in the exchanger 111. This loop in which theCO2-lean gas is expanded can be repeated as many times as necessary.

Following the exchanger 111, the CO2-lean gas 46 is heated up in theexchanger 109. Finally, the outgoing fluid 48 is processed as the fluid48 in FIG. 1.

FIG. 4 illustrates another alternative form of the near-isentropicexpansion with production of work.

The process fluid 40 is compressed by the compressor 512 which may be amulti-stage compressor. The CO2-lean gas is expanded in anear-isentropic turbine 514. The temperature of the fluid 503 mustremain above the cryo-condensation temperature for CO2.

Part of the CO2 contained in the fluid 503 then condenses in the vessel111. The solid CO2 62 is tipped into the liquid bath 121 and the nextsteps are the same as those described in FIG. 1 (from the bath 121 andstream 64 onwards). The CO2-lean gas 44 passes its cold energy to theexchangers 111 and 109. The outgoing fluid 48 is processed like thefluid 48 of FIG. 1.

FIG. 5 illustrates another alternative form of the near-isentropicexpansion with the production of work, in which the working fluid is theprocess fluid.

A near-isentropic expansion with production of work is carried out onthe fluid 42 in the turbine 612 so as to cool the fluid to a temperaturebelow the cryo-condensation temperature for CO2 and thus produce solidCO2 in the form of carbon dioxide snow together with a CO2-lean gas 602.

This expansion turbine 612 needs to be designed with a great deal ofcare. It has to be suited to the high flow rates such as those of theflue gases 40 of an industrial plant, have very good isentropicefficiency, and be resistant to potential additional erosion due to thepresence of solid CO2. To achieve this, carbon dioxide snow is allowedto be present in the rotor part of the turbine (the region containedbetween the leading edge 951 and the trailing edge 954 in FIGS. 14 and15) and is forbidden or minimized in the stator part 960 upstream of therotor part (the region contained upstream of the trailing edge of thestator vanes 950) in order notably not to cause erosion of the leadingedge of the vanes 952 of the rotor part. Put differently, it ispreferable for the CO2 to be in the vapor or supersaturated vapor statein the stator part or for it to have carbon dioxide snow nucleii thatare small enough (less than 10 μm, preferably 1 μm hydraulic diameter)to avoid eroding the rotor part.

The turbine may be a radial turbine (centripetal or centrifugal). It maybe a supersonic shockwave turbine. It may be axial.

The latter technology is the best suited to high flow rates, but doesrequire numerous successive stator and rotor stages. To avoid erosion,it will be preferable for the carbon dioxide snow to be separated outdownstream of each rotor stage before the fluid enters the next statorstage. The first two technologies have the advantage of remainingeffective for high expansion ratios (in excess of 10) thus making itpossible to avoid having to perform numerous separation operations.

Moreover, other precautions have preferably to be taken in order tocreate such a turbine:

-   -   heterogeneous nucleation (on the stator and rotor surfaces)        needs to be minimized, for example by heating some of these        surfaces or by applying special coatings;    -   nucleation needs to be delayed by eliminating compounds less        volatile than CO2 (including solid particles) before they enter        the turbine, so that they do not form nucleii encouraging the        nucleation of solid CO2;    -   the erosion resistance of the surfaces needs to be increased by        using stronger metals such as titanium or by using special        coatings or surface treatments;    -   in the case of centripetal radial turbines, it is preferable for        a sweeping gas 953 to be passed across the back of the impeller        962. This gas mixes with the expanded gas at the interface        between the stator part (vanes) and the rotor part (impeller)        and thus avoids the formation and build-up of solids behind the        impeller.

This carbon dioxide snow is then separated from the CO2-lean gas in aseparator 612 to obtain a solid comprising predominantly CO2 62 and aCO2-lean gas 44.

This separation may be performed downstream of the rotor part by causingthe fluid in the rotor part to rotate and by using the centrifugaleffect to separate a CO2-rich fraction at the periphery from a CO2-leanfraction at the center. It may also be advantageous to increase thespeed and therefore achieve an additional expansion of the fluid in aconvergent nozzle 956 (a turbine known as a Laval turbine). By reducingthe pressure before decelerating the gas the amount of solidified CO2can be increased. Most of the CO2-lean gas is recovered at the center ofthe flow 959 and most of the solid CO2 is recovered at the periphery958, mixed in with a fraction of the gas.

The benefit of a turbine for performing solid cryo-condensation is thata great deal of solid CO2 can be generated in a very small volume ascompared with indirect-exchange systems.

If necessary, an additional refrigerating cycle 191, 193 of the Rankinetype or which includes a near-isentropic expansion of a working fluidwith or without the production of work provides the separator 612 withcold energy. The solid 62 comprising predominantly CO2 is tipped intothe liquid bath 121 and the next steps are the same as those depicted inFIG. 1.

The CO2-lean gas 44 is heated up by exchange of heat with the processfluid in the exchanger 109. The fluid 605 is then compressed to apressure higher than or equal to atmospheric pressure. Finally, theoutgoing fluid 48 is processed as in FIG. 1.

FIG. 6 illustrates one embodiment with near-isentropic expansion withoutthe production of work.

The process fluid 42 is still cooled to below the cryo-condensationtemperature for CO2 in the vessel 111 to produce a cooled CO2-lean gas701. It is also possible for this vessel to be situated after the“expansion/Venturi” part 702 of the method, and will now be described.

Some of the CO2 to be captured solidifies in the form of a solidcontaining predominantly CO2 62 and is extracted from the vessel 111. Toimprove CO2 capture, the fluid 701 is made to rotate about an axis thatis substantially parallel to the direction in which it flows using asystem of fixed vanes 717.

The fluid 703 is expanded as it leaves the vanes and cools, to below thecryo-condensation temperature for CO2, without producing work. Theexpansion may take place through the Venturi effect by passing the fluidthrough a restriction 718. Solid particles comprising predominantly CO2form and are recovered at the periphery of the flow thanks to thecentrifugal effect caused by the rotation of the fluid.

A mixture 705 of solid comprising predominantly CO2 and gas isrecovered. The outgoing non-condensables 44, 46 give up their coldenergy in the exchangers 111 and 109.

The stream 705 is made up predominantly of solid, although it may benecessary to separate the residual gas from the solid in a separator731. The non-condensable part then gives up its cold energy in theexchangers 111 and 109.

The solid comprising predominantly CO2 62 is tipped into the liquid bath121 and undergoes the same steps as those described in FIG. 1.

The streams 48 are used to cool the water, in the same way as the stream50 in FIG. 1.

FIG. 7 illustrates another embodiment with near-isentropic expansionwithout the production of work.

The process fluid 40 is under pressure, for example as much as 60 bar(compression performed by the compressor 101 or by an additionalcompressor). It may potentially be more concentrated in CO2 than in theother examples, typically containing between 50 and 90% by volume.

The exchange 809 comprises the same features as the exchange 109 inFIG. 1. The exchanger 811 cools the process fluid 42 to a temperaturebelow the liquefaction temperature of CO2. From this there emerges acooled process fluid 801 which is sent to a separator 812.

A CO2-rich liquid 816 is extracted by the separator 812. The residualfluid 802 is made to rotate about an axis substantially parallel to thedirection in which it flows by a system of fixed vanes 817. It isexpanded as it leaves 803 the vanes having been rotated and cooled tobelow the cryo-condensation temperature for CO2 without producing work.The expansion may take place through a Venturi effect by passing thefluid through a restriction 818.

Solid particles comprising predominantly CO2 form and are recovered atthe periphery of the flow thanks to the centrifugal effect caused by therotating of the fluid. The stream 805 is made up predominantly of solid,although it may be necessary to separate the residual gas from the solidin a separator 841. The non-condensables 44 give up their cold energy inthe exchangers 811 and 809.

In order to improve the level of CO2 capture, a second (or even a thirdor more) step in which the fluid 806 undergoes a near-isentropicexpansion with Venturi effect may be added. This step is identical tothe previous one:

-   -   the fluid 806 is made to rotate about an axis substantially        parallel to the direction in which it flows using a system of        fixed vanes 807;    -   after it has been made to rotate, the fluid leaving the vanes        808 is expanded to cool it to below the cryo-condensation        temperature for CO2 without the production of work. The        expansion may take place through a Venturi effect by passing the        fluid through a restriction 822.

The solid 62 comprising predominantly CO2 recovered at the outlet fromthe separators 841 and possibly 851 is tipped into the liquid bath 121and processed as in FIG. 1. Streams 48 are used to cool the water, inthe same way as the stream 50 in FIG. 1.

FIG. 8 depicts a plant for producing the electricity from coal,employing various units 4, 5, 6 and 7 for purifying the flue gases 19.

A primary airflow 15 passes through the unit 3 in which the coal 15 ispulverized and carried along toward the burners of the boiler 1. Asecondary airflow 16 is applied directly to the burners in order toprovide additional oxygen needed for near-complete combustion of thecoal. Feed water 17 is sent to the boiler 1 to produce steam 18 which isexpanded in a turbine 8.

The flue gases 19 resulting from the combustion, comprising nitrogen,CO2, water vapor and other impurities, undergo various treatments toremove some of said impurities. The unit 4 removes the NOx for exampleby catalysis in the presence of ammonia. The unit 5 removes dust, forexample using an electrostatic filter, and the unit 6 is adesulfurization system for removing the SO2 and/or SO3. The units 4 and6 may be superfluous depending on the composition of the productrequired. The purified flow 24 from the unit 6 (or 5 if 6 is notpresent) is then sent to a low-temperature cryo-condensationpurification unit 7 to produce a relatively pure flow 26 of CO2 and anitrogen-enriched residual flow 25. This unit 7 is also known as a CO2capture unit and implements the method that forms the subject of theinvention, as illustrated, for example in FIGS. 1 to 7.

FIG. 9 depicts a plant for producing electricity from coal, implementingvarious units 5 and 7 for purifying the flue gases 19.

A primary airflow 15 passes through the unit 3 where the coal 15 ispulverized and carried along toward the burners of the boiler 1. Asecondary flow of oxidant 16 is supplied directly to the burners inorder to provide the additional oxygen needed for near-completecombustion of the coal. This secondary oxidant is the result of themixing of flue gases 94 recirculated using a blower 91 with oxygen 90produced by a unit 10 for separating air gases. Feed water 17 is sent tothe boiler 1 to produce steam 18 which is expanded in a turbine 8.

The flue gases 19 from the combustion of the coal, comprising nitrogen,CO2, water vapor and other impurities, undergo various treatments toremove some of said impurities. The unit 5 (ESP) removes the dust, forexample using an electrostatic filter. The dust-free flow 24 from theunit 5 is sent to a low-temperature cryo-condensation purification unit7 to produce a relatively pure flow 26 of CO2 and a nitrogen-enrichedresidual flow 25. This unit 7 is also known as a CO2 capture unit andimplements the method that forms the subject of the invention, asillustrated, for example, in FIGS. 1 to 7.

In this case, the presence of a unit for separating the air gases isused to provide cold at low level for the solid cryo-condensation of CO2in the unit 7 and to carry out cryo-condensation, preferably by directexchange with the process gas. The fluid 93 may be in liquid, gaseous ordiphasic form and consists of a mixture of cooled air gases. Forexample, this may be cold gaseous nitrogen or air (at between −56° C.and −196° C.), or alternatively liquid nitrogen or air. It is intendedto be introduced into the vessel referenced 111 in FIGS. 1 to 4 and inFIG. 6, referenced 612 in FIG. 5, 731 in FIGS. 6, and 841, 851 in FIG.7.

The unit 7 may also produce a fluid 92 which will be used in the unitfor separating air gases. This may, for example, be a fraction of thelean gas leaving the vessel 111 in FIGS. 1 to 4 and 6, 612 in FIG. 5,731 in FIGS. 7 and 841, 851 in FIG. 8. This lean gas in some wayrestores cold to the unit 10 at a temperature level higher than thatafforded from the unit 10 by the fluid 93. It is advantageous for theflow rate of this injection of fluid 93 to be varied over time. Forexample, liquid nitrogen may be produced and stored by night, whenenergy is available and inexpensive and the liquid nitrogen may then beinjected by day in order to reduce the energy consumption. The time atwhich the cold is produced by the unit 10 (for example liquid nitrogen)is separated from the time at which it is used in the unit 7. In such acircumstance, the near-isentropic expansion of a gas can be carried outin the unit 10 rather than in the unit 7.

This scheme may prove well suited to instances where existing plants arebeing modified, where replacing the primary air sent to the coalpulverizers with a mixture of recirculated flue gases plus oxygen couldprove complicated, partly because of the increase in water content, theflue gases containing far more water than damp air, and partly forsafety reasons, although that should not be overestimated.

Moreover, it may prove advantageous to combine the units 7 and 10 into asingle unit, notably by carrying out one (or more) exchange(s) of heatbetween fluids of the 2 units.

FIG. 10 schematically depicts the use of a method according to theinvention in a steel-making plant. A unit 10 for separating the airgases supplies oxygen 90 to a blast furnace 900 into which iron ore 901and carbon products 902 (coal and coke) are also introduced. The blastfurnace in that instance operates in the presence of little nitrogen.

The blast furnace gases 903 made up for example of 47% CO, 36% CO2, 8%N2 and 9% other compounds such as H2 and H2O can be split into two. Most905 goes to the CO2 capture unit with another proportion 904 used toreduce the nitrogen concentration in the loop. The fluid 905 is cooledbeforehand in a direct-contact exchanger 906, has its dust removed inthe filter 103, and is then compressed by a compressor 901, is cooled inan exchanger 105 and dried in a drier 107 before entering thelow-temperature exchanger 109 where it will be cooled and then partiallyliquefied to a temperature close to the triple point for CO2 without theformation of solid. The diphasic gas-liquid fluid 912 obtained isseparated into a gaseous fraction 502 and a liquid fraction 920 in theseparator 928. The gaseous fraction 502 is then cooled bynear-isentropic expansion, for example in a turbine 514, so as to obtaina diphasic gas-solid fluid 503. This is separated in the vessel 111 intoa gaseous fraction 44 and a CO2-rich solid fraction 62. The solidfraction 62 is compressed, for example by an endless screw and mixedwith the liquid 920 in the bath 121, which is heated by gas 72 producedby vaporizing liquid 74 in the exchanger 109. The liquid CO2 64 iscompressed by a pump 69 to obtain a pressurized liquid 70 and is heatedup in the exchanger 109 without undergoing vaporization orpseudo-vaporization if the pressure is above the supercritical pressure.The lean gas is successively heated up by a compressor 315 and by theexchanger 109.

The invention may also be adapted to types of blast furnace operating onenriched air, for example by adding a CO/N2 separation using cryogenicdistillation, cooling the gas 44 to the required temperature.

FIG. 11 schematically depicts the use of a method according to theinvention in a plant for producing synthesis gas from an oxygen process(partial oxidation, gasification, auto-thermal reformer, etc.). A unit10 for separating air gases supplies oxygen 90 to a reactor 900 intowhich a carbon product 902 (coal, natural gas, biomass, household waste,etc.) is introduced.

The synthesis gases 903 chiefly comprise the compounds CO, CO2, H2 andH2O. The CO can be converted (in a so-called shift reaction) into CO2and H2 in the presence of water vapor: CO+H2O<−>CO2+H2. The fluid 905may possibly have its dust removed in a filter 103, then be compressedby a compressor 101, cooled in an exchanger 105 and dried in a dryer 107before entering the low-temperature exchanger 109 where it may bepartially liquefied at a temperature close to that of the triple pointfor CO2. This diphasic gas-liquid fluid 912 is separated into a gaseousfraction 502 and a liquid fraction 920 in the separator 928. The gaseousfraction 502 is then cooled by near-isentropic expansion, for example ina turbine 514, to obtain a diphasic gas-solid stream 503. This isseparated into a gaseous fraction 44 and a CO2-rich solid fraction 62 inthe vessel 111. The solid fraction 62 is mixed with the liquid 920 inthe bath 121, which is heated with gas 74 produced by the vaporizing ofthe liquid 72 in the exchanger 109. The liquid CO2 64 is compressed by apump and heated up in the exchanger 109 without vaporizing, orpseudo-vaporizing if the pressure is above the supercritical pressure.The lean gas 44 is successively heated up via a compressor 924 and theexchanger 109. This lean gas essentially consisting of hydrogen may besent to a gas turbine to be combusted without the emission of CO2. Theunit 10 may supply hot nitrogen 90 a which is introduced downstream ofthe dryers 910, and/or cold nitrogen 90 b, introduced directly into thevessel 111 to increase the amount of CO2 captured. In the firstinstance, the expansion in the turbine 514 of the hot nitrogen presentin the stream 502 provides additional cold energy for solidcryo-condensation of CO2 in the turbine 514; in the second instance, thecold nitrogen 90 b, by heating up upon contact with the fluid 503, leadsto solid cryo-condensation of the CO2. The other benefit of hot nitrogen90 a is that it increases the molecular weight of the gas 502, somethingthat may prove advantageous in reducing the cost of the expansion 514and/or of the compression 924. What actually happens is that when thesegases are very rich in hydrogen, it is not easy for these gases to becompressed/expanded using the technologies best suited to high flowrates, namely technologies of the axial, radial or supersonic shockwavetype. It then becomes necessary to use technologies of thepositive-displacement type, for example using pistons or screws, whichare very expensive to implement.

FIG. 12 schematically depicts the use of a method according to theinvention in a plant producing synthesis gas from steam reforming. Acarbon product 902 (natural gas, methanol, naphtha, etc.) is introducedinto a reactor 900.

The synthesis gases 903 produced in the reactor 900 chiefly comprise thecompounds CO, CO2, H2 and H2O. The fluid 905 may potentially becompressed by a compressor 101, cooled in an exchanger 105 and dried ina dryer 107 before entering a low-temperature exchanger 109 where it maybe partially liquefied at a temperature close to that of the triplepoint for CO2. The diphasic gas-liquid fluid 912 obtained is separatedinto a gaseous fraction 502 and a liquid fraction 920 in the separator928. The gaseous fraction 502 is then cooled by a near-isentropicexpansion, for example in a turbine 514, so as to obtain a gas-soliddiphasic mixture 503. This is separated into a gaseous fraction 44 and aCO2-rich solid fraction 62 in the vessel 111. The solid fraction 62 ismixed with the liquid 920 in the bath 121, which is heated with gas 74produced by the vaporizing of the liquid 72 in the exchanger 109. Theliquid CO264 is compressed by a pump and heated up in the exchanger 109without vaporizing or pseudo-vaporizing if the pressure is above thesupercritical pressure. The lean gas 44 can then be purified in terms ofCO2 at a low temperature, for example by adsorption using a molecularsieve 13X before being introduced into a cryogenic unit 924 for theproduction of CO. This unit operates, for example, by methane scrubbingor partial condensation of the CO. This unit 924 produces ahydrogen-enriched gas 929 and a CO-enriched gas 925. One or more fluidsof this unit may be compressed at low temperature, then reintroducedinto the heat exchanger 926.

In this case, solid cryo-condensation replaces elimination of CO2 byabsorption with amines (MDEA or MEA). If there is a desire to producepure hydrogen, then it is possible to add an H2 PSA into this schemeeither upstream of this solid cryo-condensation purification, that is tosay on the outlet side of the reformer 900 after the cooling of thesynthesis gas, or on the H2-rich gas 929.

It might be supposed that these solid cryo-condensation methods aredeficient in cold. In actual fact, this is not the case at all. On thecontrary, these solid cryo-condensation methods with near-isentropicexpansion of the process gas produce excessive amounts of cold,especially if the method also provides external work. The problem isthen that the CO2-rich fluids and the CO2-lean gas exit at lowtemperature, which represents an appreciable energy loss. In order tominimize the energy consumption of this method, one or more of thefollowing operations may be carried out:

-   -   internally:        -   cold compression of one of the fluids of the            cryo-condensation method:            -   process gas cooled to low temperature prior to                compression;            -   CO2-lean gas that is compressed at low temperature (cf.                FIG. 2). It can then either be expanded again or                compressed under vacuum to return it to atmospheric                pressure or it can be expanded after it has been heated                in the hot part of the method that produced the process                gas;        -   indirect solid cryo-condensation in an exchanger;    -   externally:        -   cold compression of any fluid of the plant;        -   production of liquid nitrogen and/or liquid air;        -   transcritical Rankine cycle on the CO2

FIG. 13 schematically depicts the use of a method according to theinvention implementing a transcritical Rankine cycle on the CO2. It alsoincludes the features of a method in which a liquid cryo-condensationand then a solid cryo-condensation are performed in succession and inwhich the purity of the CO2 produced is improved using two distillationcolumns, one of them to eliminate the compounds less volatile than CO2(NO2 or N2O4, SO2, etc.) and another to eliminate the compounds that aremore volatile.

The fluid 24 consists of flue gases and may be at a temperature of theorder of 150° C. and is injected into a filter 103 to remove theparticles down to a concentration level of below 1 mg/m³, preferablybelow 100 μg/m³.

The dust-free fluid 30 is cooled to a temperature of close to 0° C.,generally of between 0° C. and 10° C., so as to condense the water vaporit contains. This cooling is carried out in a tower 105 b, with waterinjected at two levels, cold water 36 b and water 34 b at a temperatureclose to the wet bulb temperature of the ambient air. It is alsopossible to conceive of indirect contact. The tower 105 may or may nothave packings. This tower may also serve as a scrubbing tower for theSO2.

On leaving this first tower, the fluid that may have been desaturated,is compressed to a pressure of between 5 and 50 bar abs in thecompressor 101. The fluid 32 is cooled to a temperature close to 0° C.and generally of between 0° C. and 10° C. so as to condense the watervapor it contains. This cooling is carried out in a tower 105 with waterinjected at two levels, cold water 36 and water 34 at a temperatureclose to the wet bulb temperature of the ambient air. It is alsopossible to conceive of indirect contact. The tower 105 may or may nothave packings.

The fluid 38 is sent to a unit 107 that eliminates the residual watervapor, for example using one and/or another of the following methods:

-   -   Adsorption on fixed beds, fluidized beds and/or rotary dryer, it        being possible for the adsorbent to be activated alumina, silica        gel or a molecular sieve (3A, 4A, 5A, 13X, etc.);    -   Condensation in a direct-contact or indirect-contact exchanger.

The process fluid 40 is cooled then brought into contact in adistillation column 79 with pure CO2, so as to recover the compoundsless volatile than CO2 in the form of a liquid containing CO2 and, forexample, NO2 (or its dimer N2O4). This liquid can be pumped andvaporized in the unit 78, then sent either to a combustion chamber toreduce the NO2 or to the unit for purifying the stream 30 bylow-pressure scrubbing of the SO2, where it acts as a reagent, eitherdirectly in the form of NO2 or in the form of nitric acid having beenreacted with water.

The process fluid 74 a is then cooled and partially condensed intoliquid form and sent to the separator 76. The liquid fraction 76 a issent to the bath 121. The gaseous fraction 76 b is sent to an expansionturbine so as there to produce a gas-solid diphasic stream 42 which isthen sent to the vessel 111 where it is separated into a CO2-lean gas 44and solid CO2 62. An auxiliary fluid 93, for example from an air gasseparation unit, may potentially supply additional cold for solidcryo-condensation. When it does, it may be advantageous to tap from theCO2-lean gas 44 a fluid 92 which returns to the unit that supplied thefluid 93. The solid 62 is compressed for example by an endless screw andinjected into the bath 121 of liquid CO2, from which a liquid 64 istapped. This liquid may potentially be pumped and introduced into adistillation column 75 where its compounds more volatile than CO2 areeliminated. The pure liquid 68 is heated up without vaporizing orpseudo-vaporizing if it is supercritical. It may once again be pumped toobtain the fluid 5 ready for transport. A part of the fluid 5 may betapped off to be vaporized or pseudo-vaporized in a unit 72. This unit72 is, for example, any arbitrary source of heat of the plant thatproduces the process fluid. This part of the fluid 80 is then expandedin a turbine 73 used to produce electricity or mechanical power and isthen cooled in the exchanger 109 and condensed by direct exchange in thebath 121, at the same time melting the solid CO2.

On leaving the exchanger 109, the fluid 48 can still notably be used toregenerate the unit that eliminates residual vapor 107 and/or forproducing cold water 36 a by evaporation in a direct-contact tower 115into which a dry fluid 50 is introduced and becomes saturated withwater, vaporizing part of it. Potentially, the cold water may undergoadditional cooling in a refrigerating unit 119. Thereafter, this coldwater can be used in one and/or other of the towers 105 and 105 b tocool the process gas before and/or after compression.

FIGS. 14 and 15 depict a turbine for carrying out near-isentropicexpansion of the process fluid with the production of external work inaccordance with the invention. The upstream stator part 960 begins withthe volute (not depicted) followed by vanes 950 which may be fixed orvariable. Next comes the rotor part 960 which, for example, comprisesblades 952 with a leading edge 951 where the rotor part 960 begins and atrailing edge 954 where it ends.

Downstream of the rotor part, if centrifugal force is not to be used onthe solid parts, the rotor part may consist of a simple decelerationcone.

If the downstream stator part 961 is to be used to achieve a firstseparation, then the fact that the fluid has been made to rotate in therotor part and the centrifugal effect can be used to separate a CO2-richfraction at the periphery from a CO2-lean fraction at the center. It mayalso be advantageous to increase the speed and therefore perform anadditional expansion of the fluid in a convergent nozzle 956 (aso-called “Laval” turbine). By reducing the pressure before deceleratingthe gas, the amount of solidified CO2 can be increased. Most of theCO2-lean gas is recovered at the center of the flow 959 and most of thesolid CO2 is recovered at the periphery 958, mixed in with a fraction ofgas.

1-13. (canceled)
 14. A method for producing at least one CO2-lean gasand one or more CO2-rich primary fluids from a process fluid containingCO2 and at least one compound more volatile than CO2, comprising: a) afirst cooling of said process fluid by exchange of heat with no changein state; b) a second cooling of at least part of said process fluidcooled in step a) so as to obtain at least one solid containingpredominantly CO2 and at least said CO2-lean gas; and c) a stepcomprising liquefaction and/or sublimation of at least part of saidsolid and making it possible to obtain said one or more CO2-rich primaryfluids; wherein step b) is performed in at least one expansion turbine,said solid forming inside said turbine.
 15. The method of claim 14,wherein said solid is in the form of carbon dioxide snow.
 16. The methodof claim 14, wherein said expansion turbine comprises at least one rotorpart and at least one stator part situated upstream of said rotor part,and in that step b) is performed in said rotor part.
 17. The method ofclaim 16, wherein said process fluid comprising CO2 and at least onecompound more volatile than CO2 remains in the vapor state in saidstator part.
 18. The method of claim 16, wherein said process fluid isin the supersaturated vapor state in said stator part situated upstreamof said rotor part.
 19. The method of claim 16, further comprising, astep b1), after step b), in which said solid comprising predominantlyCO2 and said CO2-lean gas that were obtained in step b) are separatedinto at least one CO2-rich fraction and a CO2-lean fraction), said stepb1) being performed in a region situated downstream of said rotor part.20. The method of claim 19, wherein in step b), a rotational movement isimparted to said solid comprising predominantly CO2 and to said CO2-leangas and in that, in step b1), the separation of said solid containingpredominantly CO2 from said CO2-lean gas, both obtained in step b),comprises a separation through a centrifugal effect induced by therotational movement imparted, in step b), to said solid comprisingpredominantly CO2.
 21. The method of claim 14, wherein certain parts ofsaid turbine are heated.
 22. The method of claim 14, wherein saidturbine comprises surfaces that are polished and/or that are coated witha given material aimed at limiting heterogeneous nucleation of saidsolid comprising predominantly CO2 on said surfaces and/or on saidcoating material.
 23. The method of claim 14, wherein said process fluidcomprises compounds less volatile than CO2 and in that said methodcomprises, prior to step b), one or more steps of purifying said processfluid to remove said less volatile compounds.
 24. The method of claim14, wherein said turbine comprises titanium.
 25. The method of claim 14,wherein a sweeping gas is injected into said turbine, said sweeping gascoming into contact with said process fluid.
 26. The method of claim 14,wherein said turbine has a degree of reaction in excess of 50%.